The tool manufacturer Sandvik also has a decent free online set of courses that I found helpful.[0] I have also heard people mention the Titans of CNC online training courses,[1] although I have not checked them out personally.
Finally, "Machinery's Handbook" is also a venerable resource that deserves mention, but more a desk reference than something to read through as a learner. They come out with new editions regularly, but older ones are still very useful (and have better quality bindings).
Overall I would recommend seeking out resources that are addressing a professional audience, rather than a hobbyist one. Hobby-level discourse and instruction about machining varies widely in quality.
[0] https://www.sandvik.coromant.com/en-us/services/education/pa...